The glaciated mountain watersheds of Southeast Alaska are rapidly changing with some of the highest documented rates of glacier volume loss on Earth reported to date. With glacial retreat comes a shift in the seasonal runoff pattern from a predictable hydrograph driven by summertime glacial melt to a potentially more stochastic regime influenced by rainfall and snowmelt. The stable isotope ratios of stream water are commonly used in watershed-scale studies to trace different water source contributions to streamflow. This technique is particularly in watersheds with glaciers because snow and glacier ice melt are typically depleted in δ18O compared to other waters sources, such as rainfall and groundwater. Here we analyzed stream water isotopes (δ18O) and stream discharge from five watersheds in Southeast Alaska collected between 2018-2024. The sites sampled ranged in glacial watershed coverage from 0 to 55% and were sampled approximately twice monthly during the peak glacial runoff season of May through October. Our goal was to examine seasonal patterns in δ18O across sites and years. The sites with higher glacial coverage were more depleted in δ18O compared to sites with less watershed ice cover and showed a seasonal depletion corresponding to the period of peak runoff, indicating an increased contribution of glacial meltwater relative to other water sources. Sites characterized by low glacial cover showed greater variation in source water over the season as reflected by the coefficient of variation values. These differences and the variation in source water emphasize that glacier retreat could bring greater stochasticity in terms of source water and seasonal patterns in runoff to these coastal watersheds.